Firstly, this invention relates to a method of removing fibrous shells from cereal grains, the method comprising a step of exposure of the cereal grains to a thermal shock by a cryogenic medium and a step of mechanical treatment thereof.
A method of this kind is known from Belgian patent 902 584, for example. In this known method of separating the exterior shell layer or layers from the remaining portion of legumes and cereals, the agricultural products to be treated are cooled or frozen, preferably using liquid nitrogen, whereafter they are subjected to a mechanical treatment in order to separate the exterior layer or layers from the remaining portion. Bilobated legumes fall further apart into their two lobes by these treatment steps.
Furthermore a similar cooling step using for example liquid nitrogen for separating the shells from cereal grains is known from DE-A-2 938 635, by which it is intended that only the decorticated grains are exposed to a milling treatment after separation and that the inert (nitrogen) gas atmosphere reduces the risk of explosion.
FR-A-2 032 032 discloses a method of removing the shells from seeds, in particular oil-containing seeds, like mustard seeds, wherein this removal is carried out at a low temperature, where the fats are in a solid (solidified) state. An improvement of this prior art method is known from U.S. Pat. No. 4,090,669, wherein the seeds are subjected to a thermal shock in a fluid bed, preferably using a cryogenic medium.
U.S. Pat. No. 4,436,757 discloses a method of removing the shells of sunflower seeds (decorticating) and the separation (hulling) thereof from the xe2x80x9cmeatxe2x80x9d, wherein the seeds are immersed in a bath of liquefied gas like liquid nitrogen for 1 to 60 minutes, and directly afterwards the seeds thus treated are contacted with a liquid or other aqueous heating medium having a temperature, which is at least 100xc2x0 F. higher than the boiling point of the liquefied gas.
In general dry methods for removing the shells from cereals, legumes, seeds and the like are preferred to wet methods, which are applied conventionally, wherein large amounts of water are required, as explained hereinbelow.
In the cereals processing industry, e.g. in the processing of wheat, corn, soy and tapioca into fractions containing the different constituents of the cereals, traditionally the non-usable materials like foreign matter and broken grains are separated in a first step (xe2x80x9ccleaningxe2x80x9d) by means of screening on a vibrating table, optionally using a forced flow of air and electromagnets in order to remove metal parts. In such a separation step the cleaned cereal grains to be processed further are separated from the non-usable fraction based upon differences in size and/or weight. A drawback thereof is the limited accuracy which can be achieved in such a separation. The cereal grains, from which the foreign matter has been removed, is used as starting material for further xe2x80x9cwetxe2x80x9d processing. Hereinbelow an example of the wet processing of corn into fractions of gluten and starch respectively is described in detail.
After screening of the foreign matter and broken grains from the corn, in the wet process this corn is mixed with a certain quantity of water (approximately 1,5 time the weight of corn), which if desired contains a small amount of sulphurdioxyde, and is steeped therein for a few days (xe2x80x9csteepingxe2x80x9d) and subsequently milled into a slurry such that the germs are not damaged. The slurry thus obtained is passed over screen bendings and through hydrocyclones in order to remove the germs from the slurry. The germs separated are dewatered and dried. The slurry, from which the germs have been removed, is milled again and passed over screen bendings having smaller meshes in order to remove the fibres, which are predominantly derived from the shell of the corn kernels. The fibres are washed in countercurrent with water in order to limit the loss of starch and to recover the starch in this water. After this washing step the fibres are dewatered and dried with the aid of conventional techniques, and stored.
The slurry, which now consists primarily of granules of starch and gluten and water, is separated into a fraction of starch and a fraction of gluten. This separation is carried out in centrifuges and hydrocyclones, into which water is fed in countercurrent. The gluten fraction thus obtained is dewatered and dried and milled to the desired dimensions. The starch fraction is subjected to a refining treatment with acid and/or enzymes in order to obtain all sorts of compositions of glucose syrups. If desired, the starch can be modified into more specific derivatives thereof.
One of the serious disadvantages of these traditional xe2x80x9cwetxe2x80x9d methods of processing is the large volume of water, which is consumed and which has to be removed subsequently from the separated fractions such as the germs, fibres and gluten, by means of dewatering and drying, for which operations a large need for energy exists. Furthermore the process water, if it cannot be reused in other parts of the plant, has to be recognized as industrial waste water, which may not be discarded of as such via the sewer, so that high additional costs are involved in the disposal and processing of this kind of water.
Although the dry methods mentioned above using a cryogenic medium, wherein the shell is removed while the grains are deeply cooled, do not suffer from the disadvantages involved in the wet processing regarding drying and dewatering, respectively waste water, and from that point of view look very promising, these methods have not been used on an industrial scale as far as known in the processing of cereal grains into individual fractions of starch and gluten respectively. In this regard it has to be noted that in the cereal processing industry a distinction is made between on the one hand wet processes (xe2x80x9cwet millingxe2x80x9d), wherein the separation of the cereal grains into the different constituents thereof, such as starch, gluten, germs and the like is aimed for, which constituents are suitable for different end purposes, and on the other hand dry processes (xe2x80x9cdry millingxe2x80x9d), wherein such a separation into the different constituents is not intended, but instead thereof a flour is obtained, which is composed of all constituents of the cereal grains.
Firstly, the object of the present invention is to provide an improved method for the processing of cereal grains into starch and gluten, wherein the shells of the cereal grains are removed in an efficient manner at a relatively low need for water and energy.
A further object of the invention is to provide substeps, suitable in the processing of cereal grains into starch and gluten, wherein almost no water or no water at all is required.
The method of removing fibrous shells from cereal grains according to the invention comprises a step of exposure of the cereal grains to a thermal shock by a cryogenic medium and a step of mechanical treatment thereof, wherein the method also comprises a pretreatment step, wherein the cereal grains are subjected to a moistening treatment.
It has been found that when cereal grains, which normally contain a relatively low moisture content in the range of 10-18% after harvesting and at storage, are allowed to absorb water for a sufficient period of time, and the cereal grains thus moistened are exposed to a thermal shock by means of a cryogenic medium, then the shell very easily splits off from the remainder of the grain by and during the subsequent mechanical operation. When the pretreatment according to the invention is applied, furthermore it has appeared that the cereal grains do not need to be cooled deeply, with the result that the exposure time to the cryogenic medium can be retained low, which has a beneficial influence on the process and/or production rate. If the cereal grains are exposed to the cryogenic medium for a too long period of time, so that these are thoroughly cold, there is a smaller amount of large fibres and a larger amount of broken germs, which is undesired in view of the subsequent processing steps.
It is believed that moisture absorbed during the pretreatment step, e.g. by allowing the grains to steep in water for a sufficient period of time, enhances the strains and stresses, which are generated in the shell as the water freezes rapidly therein, and the cereal grains thus treated are subjected to a mechanical operation.